Fossils Put Dent in Geoengineering Claims

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During Earth's last ice age, iron dust dumped into the ocean
fertilized the garden of the sea, feeding a plankton bloom that
soaked up carbon dioxide from the air, a new study confirms.

But the results deal a blow to some geoengineering schemes that
claim that people may be able use
iron fertilization to slow global warming. The planet's
natural experiment shows it would take at least a thousand years
to lower carbon dioxide levels by 40 parts per million — the
amount of the drop during the ice age.

Meanwhile, carbon dioxide is now increasing by 2 parts per
million yearly, so in about 20 years human emissions could add
another 40 parts per million of carbon dioxide to the
atmosphere. Levels currently hover around 400
parts per million.

"Even if we could reproduce what works in the natural world, it's
not going to solve the carbon
dioxide problem," said Alfredo Martínez-García, a climate
scientist at ETH Zurich in Switzerland and author of the study,
published today (March 20) in the journal Science.

Iron and ice

The idea of fertilizing the ocean with iron to combat rising
carbon-dioxide levels has intrigued scientists for more than 20
years, since the late researcher John Martin observed that the
ice-age drop in carbon dioxide (noted in ice cores) synced with a
surge in iron-rich dust.

The link between more iron in the ocean and less carbon dioxide
in the air lies in the tiny ocean-dwelling plants called
phytoplankton. For them, iron is an essential nutrient. In
some regions, such as the Southern Ocean surrounding Antarctica,
the water lacks iron but has plenty of the other nutrients that
phytoplankton need to grow. Sprinkling a little iron dust in that
region could boost plankton numbers considerably, the theory
goes.

When climate changes during the ice age boosted the amount of
iron-rich dust blowing into the Southern Ocean, the phytoplankton
there grew and spread, gobbling up more carbon dioxide from the
atmosphere in the process, Martin said.

The model, called the iron fertilization hypothesis, has been
borne out by modern tests. Seeding small areas of the oceans
does, indeed, cause big phytoplankton growth spurts. [ 7
Schemes to Geoengineer the Planet ]

In the new study, Martínez-García and his co-authors examined
seafloor sediments from the Subantarctic Zone of the Southern
Ocean, southwest of Africa. When the last ice age peaked between
26,500 and 19,000 years ago, dust blowing off of Patagonia and
the southern part of South America settled there, the drill core
shows.

To gauge the changes in seawater composition at the time, the
researchers examined the fossilized shells of microscopic marine
animals called foraminifera, which eat plankton and preserve the
local ocean chemistry in their shells. During the ice age,
nitrogen
levels dropped when iron-rich dust increased at the drill core
site, Martínez-García discovered.

"It is particularly gratifying to see such persuasive evidence
for the iron hypothesis now appear in the sediment record," said
Kenneth Coale, director of the Moss Landing Marine Laboratories
in Moss Landing, Calif., who was not involved in the study.

In previous research, Coale and colleagues looked at the effect
of iron enrichment in these waters for over 40 days. The new
study shows "the effects of iron enrichment for over 40,000
years, providing a historical validation of the iron hypothesis,"
Coale said.

Too big to succeed?

The dust level in the drill core suggests that about four to
fives times more sediment fell across the Southern Ocean between
South America and Africa during the ice
age than the amount that falls there today, Martínez-García
said.

"The magnitude of the area we are talking about is equivalent to
three times the areas of the entire United States, and is
maintained for several thousand years," he told Live Science.
"This helps put into perspective what we can do in terms of the
modern ocean."

The new study supported the argument that the amount of iron
needed for
geoengineering is untenable in the long term, said Gabriel
Filippelli, a biogeochemist at Indiana University-Purdue
University in Indianapolis. "It is difficult to imagine even a
decade-long international effort of iron fertilization, sustained
by continual ship runs dumping iron in a weather-hostile and
isolated region of the world, let alone an effort that lasts a
millennium," Filippelli said.

But Filippelli also said he thinks the ice-age iron story is more
complicated than just dust blowing in the wind. "The authors note
only one source of iron — from above," he said. There is also
evidence that the oceans were richer in iron because of more
river input during the ice ages, he said. Thus, the ice-age ocean
had extra iron from above and from below.